A computer may include a main unit and a removable unit which is removable from the main unit. When a user code is transmitted to the main unit from the removable unit, the main unit compares the contents of the user code with the contents of a unit code which is recorded in the main unit. When the contents of the user code agree with the contents of a unit code, the main unit allows a system to start up. When the contents of the user code do not agree with the contents of a unit code, the main unit does not allow the system to start up.    Patent Document: Japanese Laid-Open Patent Publication No. 2002-341957